In SQL, both UNION and UNION ALL are used to combine the results of two or more SELECT queries. However, there are key differences between the two, particularly in how they handle duplicates and their impact on performance. Below, we will explore these differences in detail.
1. UNION
- Removes Duplicates:
UNION combines the results of multiple SELECT queries, but it removes any duplicate rows in the result set. Only distinct rows will appear in the final output. - Performance Impact: Since
UNION removes duplicates by default, it has a performance cost associated with this deduplication process. The database needs to check the result set and eliminate any rows that are identical. - Syntax:
sql
SELECT column1, column2 FROM table1
UNION
SELECT column1, column2 FROM table2;
2. UNION ALL
- Includes Duplicates: Unlike
UNION, UNION ALL combines the results of multiple SELECT queries but includes all rows, even if they are duplicates. If there are identical rows in the combined queries, they will all appear in the result set. - Performance Impact: Since
UNION ALL does not remove duplicates, it is generally faster than UNION. This is because there is no need for the database to perform additional checks to remove duplicates. - Syntax:
sql
SELECT column1, column2 FROM table1
UNION ALL
SELECT column1, column2 FROM table2;
3. When to Use Each
- Use
UNION: - When you need to ensure that your result set does not contain any duplicate rows.
- This is useful when you want a distinct list of items, and you don’t want repeated values to appear in your results.
- Use
UNION ALL: - When you want to include all rows from the combined queries, even duplicates.
- This is useful when duplicates are acceptable, or when performance is a priority and you want to avoid the overhead of deduplication.
4. Example Scenario
Consider two tables, employees1 and employees2, each containing employee records with common columns: id, name, and department.
Using UNION
sql
SELECT id, name, department FROM employees1
UNION
SELECT id, name, department FROM employees2;
This query will return a list of employees, ensuring that no duplicates are included in the final result set.
Using UNION ALL
sql
SELECT id, name, department FROM employees1
UNION ALL
SELECT id, name, department FROM employees2;
This query will return all employees from both tables, including any duplicates.
5. Practical Example
Here’s a practical example to illustrate the difference. Suppose employees1 and employees2 have the following data:
employees1 Table:
idnamedepartment1AliceHR2BobIT
employees2 Table:
idnamedepartment2BobIT3CharlieMarketing
Using UNION
sql
SELECT id, name, department FROM employees1
UNION
SELECT id, name, department FROM employees2;
Result:
idnamedepartment1AliceHR2BobIT3CharlieMarketing
- The duplicate row (id = 2, name = Bob, department = IT) from both tables is removed in the result set.
Using UNION ALL
sql
SELECT id, name, department FROM employees1
UNION ALL
SELECT id, name, department FROM employees2;
Result:
idnamedepartment1AliceHR2BobIT2BobIT3CharlieMarketing
- The duplicate row (id = 2, name = Bob, department = IT) appears twice in the result set, as
UNION ALL includes all rows, even duplicates.
6. Summary
UNION: Use when you need to eliminate duplicates from the combined result set. This is useful when a distinct list of results is required.UNION ALL: Use when you want to include all rows, including duplicates. This can improve performance, especially in scenarios where duplicates are acceptable.
Understanding when to use each can significantly impact both the result set you get and the performance of your queries, so choose the one that best suits your needs.